A suicide bomber killed at least 19 people Friday in a blast inside a Shiite mosque in eastern Saudi Arabia as worshippers commemorated the 7th century birth of a revered figure, residents and officials said.

Loyalists of the extremist the Islamic State of Iraq and Syria (ISIS) claimed responsibility for the attack - the second against Shiites in the kingdom in six months. In November, the extremist ISIS group was accused of being behind the shooting and killing of eight worshippers in the eastern Saudi Arabian village of al-Ahsa.

Despite a string of ISIS-related attacks over the past several months that have also targeted police, Friday’s suicide bombing appears to be the deadliest in the country in nearly a decade.

Habib Mahmoud, managing editor for the state-linked Al-Sharq newspaper in Qatif, said that the local Red Crescent authorities confirmed to him that 19 people had been killed and 28 wounded.

Al Arabiya News Channel reported that 21 have been killed, and 81 wounded.

Saudi Arabia’s Interior Ministry has not released its count for the number of dead and wounded, but said that a suicide bomber who hid explosives under his clothes was behind the attack. Interior Ministry spokesman Maj. Gen. Mansour al-Turki said that the attacker struck the Imam Ali mosque in a village called al-Qudeeh.

In a statement distributed on Twitter feeds linked to ISIS group loyalists, a group purporting to be the ISIS branch in Saudi Arabia issued the claim. It could not be independently confirmed if the new group actually has operational links to ISIS, which is based in Syria and Iraq.

The group’s statement carried a logo in Arabic referring to itself as the “Najd Province” - a reference to the historic region that is home to the capital Riyadh and the ruling Al Saud family, as well as the ultraconservative Wahhabi branch of Islam.

Mahmoud, the editor in Qatif, said the attacker stood with the worshippers during prayer and then detonated his suicide vest as people were leaving the mosque.

A local activist, Naseema al-Sada, told The Associated Press by telephone from Qatif that the suicide bomber attacked worshippers as they were commemorating the birth of Imam Hussain, a revered figure among Shiites. She said the local hospital has called on residents to donate blood.